The application is a continuation of PCT/EP02/07835 filed on Jul. 15, 2002.
High directivity microstrip arrays are becoming an alternative to parabolic reflector antennas due to its thin profile and less mechanical complexity [J. Huang. “Ka-Band Circularly Polarized High-Gain Microstrip Array Antenna”, IEEE Trans. Antennas and Propagation, vol. 43, no. 1, pp. 113–116, January 1995.]. However, one important problem is the complexity of the feeding network to feed the large number of elements [E. Levine, G. Malamud, S. Shtrikman, D. Treves. “Study of Microstrip Array Antennas with the Feed Network”, IEEE Trans. Antennas and Propagation, vol. 37, no. 4, pp. 426–434, April 1989.]. Thus, a large space is needed for the feeding network. Furthermore, in a phased-array, phase-shifters, amplifiers and other MMICs have to be integrated together with the feeding network and this is a significant integration problem. In this sense, the present invention proposes a novel scheme for microstrip arrays using multilevel or space-filling shaped antenna elements [“Multilevel Antennae”, Invention Patent WO0122528.], [“Space-Filling Miniature Antennas”, WO0154225]. A multilevel structure for an antenna device, as it is known in prior art, consists of a conducting structure including a set of polygons, all of said polygons featuring the same number of sides, wherein said polygons are electromagnetically coupled either by means of a capacitive coupling or ohmic contact, wherein the contact region between directly connected polygons is narrower than 50% of the perimeter of said polygons in at least 75% of said polygons defining said conducting multilevel structure. In this definition of multilevel structures, circles, and ellipses are included as well, since they can be understood as polygons with a very large (ideally infinite.) number of sides. An antenna is said to be a multilevel antenna, when at least a portion of the antenna is shaped as a multilevel structure. A space-filling curve for a space-filling antenna, as it is known in prior art, is composed by at least ten segments which are connected in such a way that each segment forms an angle with their neighbours, i.e., no pair of adjacent segments define a larger straight segment, and wherein the curve can be optionally periodic along a fixed straight direction of space if and only if the period is defined by a non-periodic curve composed by at least ten connected segments and no pair of said adjacent and connected segments define a straight longer segment. Also, whatever the design of such SFC is, it can never intersect with itself at any point except the initial and final point (that is, the whole curve can be arranged as a closed curve or loop, but none of the parts of the curve can become a closed loop). The present invention consist on combining several of these elements in a novel configuration for an antenna array, such that the number of radiating elements is reduced with respect to prior art, while the overall directivity of the antenna is kept. The main advantage is that a less number of elements is needed compared to the state of the art approach when the array is designed according to the present invention. FIG. 6 shows a classical approach of a bidimensional array using circular patches where the separation between elements is less than 0.9λ at the operating frequency, being λ the wavelength of the operating frequency. FIG. 7 shows a novel scheme of a bidimensional array using a multilevel shaped patch where separation between elements is larger than 0.9λ at the operating frequency. FIG. 8 shows another novel scheme of a bidimensional array using a space-filling shaped patch where the element separation is larger than 0.9λ in one direction but less than 0.9λ in the perpendicular direction. The novel schemes presented at FIG. 7 and FIG. 8 have less number of elements compared to the classical prior-art scheme of FIG. 6. This arrangement for the arrav of using less elements is novel and constitutes the heart of the present invention. Such microstrip arrays can employ less elements thanks to the multilevel or space-filling shaped elements. An advantage of using less elements, for example, is that the feeding network complexity decreases and consequently more space is available to integrate other microwave components. Also, it reduces the antenna volume and weight, which can be an advantage in cost, for instance, in satellite antennas.
The multilevel and space-filling shaped patch elements used as a radiating elements of the array in the present invention feature high-directivity performance. Such behaviour can be found in the prior art [C. Borja, G. Font, S. Blanch, J. Romeu, “High directivity fractal boundary microstrip patch antenna”, IEE Electronic Letters, vol. 26, No. 9, pp. 778–779, 2000], [J. Anguera, C. Puente, C. Borja, R. Montero, J. Soler, “Small and High Directivity Bowtie Patch Antenna based on the Sierpinski Fractal”, Microwave and Optical Technology Letters, vol. 31, No. 3, pp. 239–241, November 2001]. The multilevel and space-filling shaped patch elements support resonating modes called fractons and fractinos according to the nomenclature heritaged from the acoustical field [B. Sapoval, Th. Gobron, A. Margolina “Vibrations of Fractal Drums”, The American Physical Society, vol. 67, No. 21, pp. 2974–2977, November 1991.]. Depending on the antenna geometry, the antenna support fracton or fractino modes: roughly speaking, such modes are resonating modes with a resonating frequency larger than the fundamental mode (the lowest resonant frequency). When the antenna is operating in a fracton or fractino mode, the directivity is much larger than the antenna when operating in the fundamental mode and even preserving a broadside radiation pattern.